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Ants are social insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from wasp-like ancestors in the mid-Cretaceous period between 110 and 130 million years ago and diversified after the rise of flowering plants. More than 12,500 out of an estimated total of 22,000 species have been classified. They are easily identified by their elbowed antennae and a distinctive node-like structure that forms a slender waist.

Ants form colonies that range in size from a few dozen predatory individuals living in small natural cavities to highly organised colonies which may occupy large territories and consist of millions of individuals. These larger colonies consist mostly of sterile wingless females forming castes of "workers", "soldiers", or other specialised groups. Nearly all ant colonies also have some fertile males called "drones" and one or more fertile females called "queens". The colonies are sometimes described as superorganisms because the ants appear to operate as a unified entity, collectively working together to support the colony.

Ants have colonised almost every landmass on Earth. The only places lacking indigenous ants are Antarctica and a few remote or inhospitable islands. Ants thrive in most ecosystems, and may form 15–25% of the terrestrial animal biomass. Their success in so many environments has been attributed to their social organisation and their ability to modify habitats, tap resources, and defend themselves. Their long co-evolution with other species has led to mimetic, commensal, parasitic, and mutualistic relationships.

Ant societies have division of labour, communication between individuals, and an ability to solve complex problems. These parallels with human societies have long been an inspiration and subject of study.

Many human cultures make use of ants in cuisine, medication and rituals. Some species are valued in their role as biological pest control agents. However, their ability to exploit resources brings ants into conflict with humans, as they can damage crops and invade buildings. Some species, such as the red imported fire ant, are regarded as invasive species, establishing themselves in areas where they are accidentally introduced.

The word ant is derived from ante of Middle English which is derived from æmette of Old English and is related to the Old High German āmeiza, hence the modern German Ameise. All of these words come from West Germanic *amaitjo, and the original meaning of the word was "the biter" (from Proto-Germanic *ai-, "off, away" + *mait- "cut"). The family name Formicidae is derived from the Latin formīca ("ant") from which the words in other Romance languages such as the Portuguese formiga, Italian formica, Spanish hormiga, Romanian furnică and French fourmi are derived. It has been hypothetized that a Proto-Indo-European word *morwi- was used, cf. Sanskrit vamrah, Latin formīca, Greek myrmex, Old Church Slavonic mraviji, Old Irish moirb, Old Norse maurr.

The family Formicidae belongs to the order Hymenoptera, which also includes sawflies, bees and wasps. Ants evolved from a lineage within the vespoid wasps. Phylogenetic analysis suggests that ants arose in the mid-Cretaceous period about 110 to 130 million years ago. After the rise of flowering plants about 100 million years ago they diversified and assumed ecological dominance around 60 million years ago. In 1966, E. O. Wilson and his colleagues identified the fossil remains of an ant (Sphecomyrma freyi) that lived in the Cretaceous period. The specimen, trapped in amber dating back to more than 80 million years ago, has features of both ants and wasps. Sphecomyrma was probably a ground forager but some suggest on the basis of groups such as the Leptanillinae and Martialinae that primitive ants were likely to have been predators underneath the surface of the soil.

During the Cretaceous period, a few species of primitive ants ranged widely on the Laurasian super-continent (the northern hemisphere). They were scarce in comparison to other insects, representing about 1% of the insect population. Ants became dominant after adaptive radiation at the beginning of the Tertiary period. By the Oligocene and Miocene ants had come to represent 20-40% of all insects found in major fossil deposits. Of the species that lived in the Eocene epoch, approximately one in ten genera survive to the present. Genera surviving today comprise 56% of the genera in Baltic amber fossils (early Oligocene), and 92% of the genera in Dominican amber fossils (apparently early Miocene).

Termites, though sometimes called white ants, are not ants and belong to the order Isoptera. Termites are actually more closely related to cockroaches and mantids. Termites are eusocial but differ greatly in the genetics of reproduction. The similar social structure is attributed to convergent evolution. Velvet ants look like large ants, but are wingless female wasps.

Ants are found on all continents except Antarctica, and only a few large islands such as Greenland, Iceland, parts of Polynesia and the Hawaiian Islands lack native ant species. Ants occupy a wide range of ecological niches, and are able to exploit a wide range of food resources either as direct or indirect herbivores, predators and scavengers. Most species are omnivorous generalists but a few are specialist feeders. Their ecological dominance may be measured by their biomass, and estimates in different environments suggest that they contribute 15-20% (on average and nearly 25% in the tropics) of the total terrestrial animal biomass, which exceeds that of the vertebrates.

Ants range in size from 0.75 to 52 millimetres (0.030–2.0 in), the largest species being the fossil Titanomyrma giganteum, the queen of which was 6 centimetres (2.4 in) long with a wingspan of 15 centimetres (5.9 in). Ants vary in colour; most ants are red or black, but a few species are green and some tropical species have a metallic lustre. More than 12,000 species are currently known (with upper estimates of about 22,000) (see the article List of ant genera), with the greatest diversity in the tropics. Taxonomic studies continue to resolve the classification and systematics of ants. Online databases of ant species, including AntBase and the Hymenoptera Name Server, help to keep track of the known and newly described species. The relative ease with which ants can be sampled and studied in ecosystems has made them useful as indicator species in biodiversity studies.

Ants are distinct in their morphology from other insects in having elbowed antennae, metapleural glands, and a strong constriction of their second abdominal segment into a node-like petiole. The head, mesosoma and metasoma or gaster are the three distinct body segments. The petiole forms a narrow waist between their mesosoma (thorax plus the first abdominal segment, which is fused to it) and gaster (abdomen less the abdominal segments in the petiole). The petiole can be formed by one or two nodes (the second alone, or the second and third abdominal segments).

Like other insects, ants have an exoskeleton, an external covering that provides a protective casing around the body and a point of attachment for muscles, in contrast to the internal skeletons of humans and other vertebrates. Insects do not have lungs; oxygen and other gases like carbon dioxide pass through their exoskeleton through tiny valves called spiracles. Insects also lack closed blood vessels; instead, they have a long, thin, perforated tube along the top of the body (called the "dorsal aorta") that functions like a heart, and pumps haemolymph towards the head, thus driving the circulation of the internal fluids. The nervous system consists of a ventral nerve cord that runs the length of the body, with several ganglia and branches along the way reaching into the extremities of the appendages.

An ant's head contains many sensory organs. Like most insects, ants have compound eyes made from numerous tiny lenses attached together. Ants' eyes are good for acute movement detection but do not give a high resolution. They also have three small ocelli (simple eyes) on the top of the head that detect light levels and polarization. Compared to vertebrates, most ants have poor-to-mediocre eyesight and a few subterranean species are completely blind. Some ants such as Australia's bulldog ant, however, have exceptional vision. Two antennae ("feelers") are attached to the head; these organs detect chemicals, air currents and vibrations; they are also used to transmit and receive signals through touch. The head has two strong jaws, the mandibles, used to carry food, manipulate objects, construct nests, and for defence. In some species a small pocket (infrabuccal chamber) inside the mouth stores food, so it can be passed to other ants or their larvae.

All six legs are attached to the mesosoma ("thorax"). A hooked claw at the end of each leg helps ants to climb and hang onto surfaces. Most queens and male ants have wings; queens shed the wings after the nuptial flight, leaving visible stubs, a distinguishing feature of queens. However, wingless queens (ergatoids) and males occur in a few species.

The metasoma (the "abdomen") of the ant houses important internal organs, including those of the reproductive, respiratory (tracheae) and excretory systems. Workers of many species have their egg-laying structures modified into stings that are used for subduing prey and defending their nests.

In the colonies of a few ant species, there are physical castes—workers in distinct size-classes, called minor, median, and major workers. Often the larger ants have disproportionately larger heads, and correspondingly stronger mandibles. Such individuals are sometimes called "soldier" ants because their stronger mandibles make them more effective in fighting, although they are still workers and their "duties" typically do not vary greatly from the minor or median workers. In a few species the median workers are absent, creating a sharp divide between the minors and majors. Weaver ants, for example, have a distinct bimodal size distribution. Some other species show continuous variation in the size of workers. The smallest and largest workers in Pheidologeton diversus show nearly a 500-fold difference in their dry-weights. Workers cannot mate; however, because of the haplodiploid sex-determination system in ants, workers of a number of species can lay unfertilised eggs that become fully fertile haploid males. The role of workers may change with their age and in some species, such as honeypot ants, young workers are fed until their gasters are distended, and act as living food storage vessels. These food storage workers are called repletes. This polymorphism in morphology and behaviour of workers was initially thought to be determined by environmental factors such as nutrition and hormones which led to different developmental paths; however, genetic differences between worker castes have been noted in Acromyrmex sp. These polymorphisms are caused by relatively small genetic changes; differences in a single gene of Solenopsis invicta can decide whether the colony will have single or multiple queens. The Australian jack jumper ant (Myrmecia pilosula) has only a single pair of chromosomes (males have just one chromosome as they are haploid), the lowest number known for any animal, making it an interesting subject for studies in the genetics and developmental biology of social insects.
The life of an ant starts from an egg. If the egg is fertilised, the progeny will be female (diploid); if not, it will be male (haploid). Ants develop by complete metamorphosis with the larval stages passing through a pupal stage before emerging as an adult. The larva is largely immobile and is fed and cared for by workers. Food is given to the larvae by trophallaxis, a process in which an ant regurgitates liquid food held in its crop. This is also how adults share food, stored in the "social stomach", among themselves. Larvae may also be provided with solid food such as trophic eggs, pieces of prey and seeds brought back by foraging workers and may even be transported directly to captured prey in some species. The larvae grow through a series of moults and enter the pupal stage. The pupa has the appendages free and not fused to the body as in a butterfly pupa. The differentiation into queens and workers (which are both female), and different castes of workers (when they exist), is influenced in some species by the nutrition the larvae obtain. Genetic influences and the control of gene expression by the developmental environment are complex and the determination of caste continues to be a subject of research. The developmental environment Larvae and pupae need to be kept at fairly constant temperatures to ensure proper development, and so are often moved around the various brood chambers within the colony.

A new worker spends the first few days of its adult life caring for the queen and young. It then graduates to digging and other nest work, and later to defending the nest and foraging. These changes are sometimes fairly sudden, and define what are called temporal castes. An explanation for the sequence is suggested by the high casualties involved in foraging, making it an acceptable risk only for ants that are older and are likely to die soon of natural causes.

Most ant species have a system in which only the queen and breeding females have the ability to mate. Contrary to popular belief, some ant nests have multiple queens while others can exist without queens. Workers with the ability to reproduce are called "gamergates" and colonies that lack queens are then called gamergate colonies; colonies with queens are said to be queen-right. The winged male ants, called drones, emerge from pupae along with the breeding females (although some species, like army ants, have wingless queens), and do nothing in life except eat and mate. Most ants are univoltine, producing a new generation each year. During the species specific breeding period, new reproductives, winged males and females leave the colony in what is called a nuptial flight. Typically, the males take flight before the females. Males then use visual cues to find a common mating ground, for example, a landmark such as a pine tree to which other males in the area converge. Males secrete a mating pheromone that females follow. Females of some species mate with just one male, but in some others they may mate with anywhere from one to ten or more different males. Mated females then seek a suitable place to begin a colony. There, they break off their wings and begin to lay and care for eggs. The females store the sperm they obtain during their nuptial flight to selectively fertilise future eggs. The first workers to hatch are weak and smaller than later workers, but they begin to serve the colony immediately. They enlarge the nest, forage for food and care for the other eggs. This is how new colonies start in most species. Species that have multiple queens may have a queen leaving the nest along with some workers to found a colony at a new site, a process akin to swarming in honeybees.
A wide range of reproductive strategies have been noted in ant species. Females of many species are known to be capable of reproducing asexually through thelytokous parthenogenesis and one species, Mycocepurus smithii is known to be all-female.

Ant colonies can be long-lived. The queens can live for up to 30 years, and workers live from 1 to 3 years. Males, however, are more transitory, and survive only a few weeks. Ant queens are estimated to live 100 times longer than solitary insects of a similar size.

Ants are active all year long in the tropics but, in cooler regions, survive the winter in a state of dormancy or inactivity. The forms of inactivity are varied and some temperate species have larvae going into the inactive state (diapause), while in others, the adults alone pass the winter in a state of reduced activity.

Ants communicate with each other using pheromones. These chemical signals are more developed in ants than in other hymenopteran groups. Like other insects, ants perceive smells with their long, thin and mobile antennae. The paired antennae provide information about the direction and intensity of scents. Since most ants live on the ground, they use the soil surface to leave pheromone trails that can be followed by other ants. In species that forage in groups, a forager that finds food marks a trail on the way back to the colony; this trail is followed by other ants, these ants then reinforce the trail when they head back with food to the colony. When the food source is exhausted, no new trails are marked by returning ants and the scent slowly dissipates. This behaviour helps ants deal with changes in their environment. For instance, when an established path to a food source is blocked by an obstacle, the foragers leave the path to explore new routes. If an ant is successful, it leaves a new trail marking the shortest route on its return. Successful trails are followed by more ants, reinforcing better routes and gradually finding the best path.

Ants use pheromones for more than just making trails. A crushed ant emits an alarm pheromone that sends nearby ants into an attack frenzy and attracts more ants from further away. Several ant species even use "propaganda pheromones" to confuse enemy ants and make them fight among themselves. Pheromones are produced by a wide range of structures including Dufour's glands, poison glands and glands on the hindgut, pygidium, rectum, sternum and hind tibia. Pheromones are also exchanged mixed with food and passed by trophallaxis, transferring information within the colony. This allows other ants to detect what task group (e.g., foraging or nest maintenance) other colony members belong to. In ant species with queen castes, workers begin to raise new queens in the colony when the dominant queen stops producing a specific pheromone.

Some ants produce sounds by stridulation, using the gaster segments and their mandibles. Sounds may be used to communicate with colony members or with other species.

Ants attack and defend themselves by biting and, in many species, by stinging, often injecting or spraying chemicals like formic acid. Bullet ants (Paraponera), located in Central and South America, are considered to have the most painful sting of any insect, although it is usually not fatal to humans. This sting is given the highest rating on the Schmidt Sting Pain Index. The sting of Jack jumper ants can be fatal, and an antivenom has been developed. Fire ants, Solenopsis spp., are unique in having a poison sac containing piperidine alkaloids. Their stings are painful and can be dangerous to hypersensitive people.

Trap-jaw ants of the genus Odontomachus are equipped with mandibles called trap-jaws, which snap shut faster than any other predatory appendages within the animal kingdom. One study of Odontomachus bauri recorded peak speeds of between 126 and 230 km/h (78 - 143 mph), with the jaws closing within 130 microseconds on average. The ants were also observed to use their jaws as a catapult to eject intruders or fling themselves backwards to escape a threat. Before the strike, the ant opens its mandibles extremely widely and locks them in this position by an internal mechanism. Energy is stored in a thick band of muscle and explosively released when triggered by the stimulation of sensory hairs on the inside of the mandibles. The mandibles also permit slow and fine movements for other tasks. Trap-jaws are also seen in the following genera: Anochetus, Orectognathus, and Strumigenys, plus some members of the Dacetini tribe, which are viewed as examples of convergent evolution. A Malaysian species of ant in the Camponotus cylindricus group has enlarged mandibular glands that extend into their gaster. When disturbed, workers rupture the membrane of the gaster, causing a burst of secretions containing acetophenones and other chemicals that immobilise small insect attackers. The worker subsequently dies. Suicidal defences by workers are also noted in a Brazilian ant Forelius pusillus where a small group of ants leaves the security of the nest after sealing the entrance from the outside each evening.
In addition to defence against predators, ants need to protect their colonies from pathogens. Some worker ants maintain the hygiene of the colony and their activities include undertaking or necrophory, the disposal of dead nest-mates. Oleic acid has been identified as the compound released from dead ants that triggers necrophoric behaviour in Atta mexicana while workers of Linepithema humile react to the absence of characteristic chemicals (dolichodial and iridomyrmecin) present on the cuticle of their living nestmates.

Nests may be protected from physical threats such as flooding and over-heating by elaborate nest architecture. Workers of Cataulacus muticus, an arboreal species that lives in plant hollows, respond to flooding by drinking water inside the nest, and excreting it outside. Camponotus anderseni which nests in the cavities of wood in mangrove habitats deals with submergence under water by switching to anaerobic respiration.

Many animals can learn behaviours by imitation but ants may be the only group apart from mammals where interactive teaching has been observed. A knowledgeable forager of Temnothorax albipennis leads a naive nest-mate to newly discovered food by the process of tandem running. The follower obtains knowledge through its leading tutor. Both leader and follower are acutely sensitive to the progress of their partner with the leader slowing down when the follower lags, and speeding up when the follower gets too close.

Controlled experiments with colonies of Cerapachys biroi suggest that individuals may choose nest roles based on their previous experience. An entire generation of identical workers was divided into two groups whose outcome in food foraging was controlled. One group was continually rewarded with prey, while it was made certain that the other failed. As a result, members of the successful group intensified their foraging attempts while the unsuccessful group ventured out less and less. A month later, the successful foragers continued in their role while the others moved to specialise in brood care.

Complex nests are built by many ants, but other species are nomadic and do not build permanent structures. Ants may form subterranean nests or build them on trees. These nests can be found in the ground, under stones or logs, inside logs, hollow stems or even acorns. The materials used for construction include soil and plant matter, and ants carefully select their nest sites; Temnothorax albipennis will avoid sites with dead ants, as these may indicate the presence of pests or disease. They are quick to abandon established nests at the first sign of threats.

The army ants of South America and the driver ants of Africa do not build permanent nests, but instead alternate between nomadism and stages where the workers form a temporary nest (bivouac) from their own bodies, by holding each other together.

Weaver ant (Oecophylla spp.) workers build nests in trees by attaching leaves together, first pulling them together with bridges of workers and then inducing their larvae to produce silk as they are moved along the leaf edges. Similar forms of nest construction are seen in some species of Polyrhachis.

Most ants are generalist predators, scavengers and indirect herbivores, but a few have evolved specialised ways of obtaining nutrition. Leafcutter ants (Atta and Acromyrmex) feed exclusively on a fungus that grows only within their colonies. They continually collect leaves which are taken to the colony, cut into tiny pieces and placed in fungal gardens. Workers specialise in tasks according to their sizes. The largest ants cut stalks, smaller workers chew the leaves and the smallest tend the fungus. Leafcutter ants are sensitive enough to recognise the reaction of the fungus to different plant material, apparently detecting chemical signals from the fungus. If a particular type of leaf is toxic to the fungus the colony will no longer collect it. The ants feed on structures produced by the fungi called gongylidia. Symbiotic bacteria on the exterior surface of the ants produce antibiotics that kill bacteria that may harm the fungi.

Foraging ants travel distances of up to 200 metres (700 ft) from their nest and usually find their way back using scent trails. Some ants forage at night. Day foraging ants in hot and arid regions face death by desiccation, so the ability to find the shortest route back to the nest reduces that risk. Diurnal desert ants of the genus Cataglyphis such as the Sahara desert ant navigate by keeping track of direction as well as distance travelled. Distances travelled are measured using an internal pedometer that keeps count of the steps taken and also by evaluating the movement of objects in their visual field (optical flow). Directions are measured using the position of the sun. They integrate this information to find the shortest route back to their nest. Like all ants they can also make use of visual landmarks when available and use other olfactory and tactile cues to navigate. Some species of ant are able to use the Earth's magnetic field for navigation. Ants' compound eyes have specialised cells that detect polarised light from the Sun, which is used to determine direction. These polarization detectors are sensitive in the ultraviolet region of the light spectrum. In some army ant species, a group of foragers that get separated from the main column can sometimes turn back on themselves and form a circular ant mill. The workers may then run around continuously until they die of exhaustion.

Worker ants do not have wings and reproductive females lose their wings after their mating flights in order to begin their colonies. Therefore, unlike their wasp ancestors, most ants travel by walking. Some species are capable of leaping. For example, Jerdon's jumping ant (Harpegnathos saltator) is able to jump by synchronising the action of its mid and hind pairs of legs. There are several species of gliding ant including Cephalotes atratus; this may be a common trait among most arboreal ants. Ants with this ability are able to control the direction of their descent while falling.

Other species of ants can form chains to bridge gaps over water, underground, or through spaces in vegetation. Some species also form floating rafts that help them survive floods. These rafts may also have a role in allowing ants to colonise islands. Polyrhachis sokolova, a species of ant found in Australian mangrove swamps, can swim and live in underwater nests. Since they lack gills, they breathe in trapped pockets of air in the submerged nests.

Not all ants have the same kind of societies. The Australian bulldog ants are among the biggest and most basal of ants. Like virtually all ants they are eusocial, but their social behaviour is poorly developed compared to other species. Each individual hunts alone, using its large eyes instead of its chemical senses to find prey.

Some species (such as Tetramorium caespitum) attack and take over neighbouring ant colonies. Others are less expansionist but just as aggressive; they invade colonies to steal eggs or larvae, which they either eat or raise as workers/slaves. Extreme specialists among these slave-raiding ants, such as the Amazon ants, are incapable of feeding themselves and need captured workers to survive. Captured workers of the enslaved species Temnothorax have evolved a counter strategy, destroying just the female pupae of the slave-making Protomognathus americanus, but sparing the males (who don't take part in slave-raiding as adults).

Ants identify kin and nestmates through their scent, which comes from hydrocarbon-laced secretions that coat their exoskeletons. If an ant is separated from its original colony, it will eventually lose the colony scent. Any ant that enters a colony without a matching scent will be attacked.

Parasitic ant species enter the colonies of host ants and establish themselves as social parasites; species like Strumigenys xenos are entirely parasitic and do not have workers, but instead rely on the food gathered by their Strumigenys perplexa hosts. This form of parasitism is seen across many ant genera, but the parasitic ant is usually a species that is closely related to its host. A variety of methods are employed to enter the nest of the host ant. A parasitic queen can enter the host nest before the first brood has hatched, establishing herself prior to development of a colony scent. Other species use pheromones to confuse the host ants or to trick them into carrying the parasitic queen into the nest. Some simply fight their way into the nest.

A conflict between the sexes of a species is seen in some species of ants with the reproductives apparently competing to produce offspring that are as closely related to them as possible. The most extreme form involves the production of clonal offspring. An extreme of sexual conflict is seen in Wasmannia auropunctata, where the queens produce diploid daughters by thelytokous parthenogenesis and males produce clones by a process where a diploid egg loses its maternal contribution to produce haploid males that are clones of the father.

Ants form symbiotic associations with a range of species, including other ant species, other insects, plants, and fungi. They are preyed on by many animals and even certain fungi. Some arthropod species spend part of their lives within ant nests, either preying on ants, their larvae and eggs, consuming the ants' food stores, or avoiding predators. These inquilines can bear a close resemblance to ants. The nature of this ant mimicry (myrmecomorphy) varies, with some cases involving Batesian mimicry, where the mimic reduces the risk of predation. Others show Wasmannian mimicry, a form of mimicry seen only in inquilines.

Aphids and other hemipteran insects secrete a sweet liquid called honeydew when they feed on plant sap. The sugars in honeydew are a high-energy food source, which many ant species collect. In some cases the aphids secrete the honeydew in response to the ants' tapping them with their antennae. The ants in turn keep predators away and will move the aphids between feeding locations. On migrating to a new area, many colonies will take the aphids with them, to ensure a continued supply of honeydew. Ants also tend mealybugs to harvest their honeydew. Mealybugs can become a serious pest of pineapples if ants are present to protect mealybugs from their natural enemies.

Myrmecophilous (ant-loving) caterpillars of the family Lycaenidae (e.g., blues, coppers, or hairstreaks) are herded by the ants, led to feeding areas in the daytime, and brought inside the ants' nest at night. The caterpillars have a gland which secretes honeydew when the ants massage them. Some caterpillars produce vibrations and sounds that are perceived by the ants. Other caterpillars have evolved from ant-loving to ant-eating: these myrmecophagous caterpillars secrete a pheromone that makes the ants act as if the caterpillar is one of their own larvae. The caterpillar is then taken into the ants' nest where it feeds on the ant larvae.

Fungus-growing ants that make up the tribe Attini, including leafcutter ants, cultivate certain species of fungus in the Leucoagaricus or Leucocoprinus genera of the Agaricaceae family. In this ant-fungus mutualism, both species depend on each other for survival. The ant Allomerus decemarticulatus has evolved a three-way association with the host plant Hirtella physophora (Chrysobalanaceae), and a sticky fungus which is used to trap their insect prey.
Lemon ants make devil's gardens by killing surrounding plants with their stings and leaving a pure patch of lemon ant trees (Duroia hirsuta). This modification of the forest provides the ants with more nesting sites inside the stems of the Duroia trees. Although some ants obtain nectar from flowers, pollination by ants is somewhat rare. Some plants have special nectar exuding structures, extrafloral nectaries that provide food for ants, which in turn protect the plant from more damaging herbivorous insects. Species like the bullhorn acacia (Acacia cornigera) in Central America have hollow thorns that house colonies of stinging ants (Pseudomyrmex ferruginea) that defend the tree against insects, browsing mammals, and epiphytic vines. Isotopic labelling studies suggest that plants also obtain nitrogen from the ants. In return, the ants obtain food from protein and lipid rich Beltian bodies. Another example of this type of ectosymbiosis comes from the Macaranga tree, which has stems adapted to house colonies of Crematogaster ants.

Many tropical tree species have seeds that are dispersed by ants. Seed dispersal by ants or myrmecochory is widespread and new estimates suggest that nearly 9% of all plant species may have such ant associations. Some plants in fire-prone grassland systems are particularly dependent on ants for their survival and dispersal as the seeds are transported to safety below the ground. Many ant-dispersed seeds have special external structures, elaiosomes, that are sought after by ants as food. A convergence, possibly a form of mimicry, is seen in the eggs of stick insects. They have an edible elaiosome-like structure and are taken into the ant nest where the young hatch.

Most ants are predatory and some prey on and obtain food from other social insects including other ants. Some species specialise in preying on termites (Megaponera and Termitopone) while a few Cerapachyinae prey on other ants. Some termites, including Nasutitermes corniger, form associations with certain ant species to keep away other predatory ant species. The tropical wasp Mischocyttarus drewseni coats the pedicel of its nest with an ant-repellant chemical. It is suggested that many tropical wasps may build their nests in trees and cover them to protect themselves from ants. Stingless bees (Trigona and Melipona) use chemical defences against ants. Army ants forage in a wide roving column attacking any animals in that path that are unable to escape. Eciton burchellii is the swarming ant most commonly attended by "ant-following" birds such as antbirds and woodcreepers.

Flies in the Old World genus Bengalia (Calliphoridae) prey on ants and are kleptoparasites, snatching prey or brood from the mandibles of adult ants. Wingless and legless females of the Malaysian phorid fly (Vestigipoda myrmolarvoidea) live in the nests of ants of the genus Aenictus and are cared for by the ants.

Fungi in the genera Cordyceps and Ophiocordyceps infect ants, causing them to climb up plants and sink their mandibles into plant tissue. The fungus kills the ant, grows on its remains, and produces a fruiting body. It appears that the fungus alters the behaviour of the ant to help disperse its spores in a microhabitat that best suits the fungus. Strepsipteran parasites also manipulate their ant host to climb grass stems, to help the parasite find mates. A nematode (Myrmeconema neotropicum) that infects canopy ants (Cephalotes atratus) causes the black coloured gasters of workers to turn red. The parasite also alters the behaviour of the ant, and makes them carry their gasters high. The conspicuous red gasters are mistaken by birds for ripe fruits such as Hyeronima alchorneoides and eaten. The droppings of the bird are collected by other ants and fed to their young leading to the further spread of the nematode.

South American poison dart frogs in the genus Dendrobates feed mainly on ants, and the toxins in their skin may come from the ants. Several South American antbirds follow army ants to feed on the insects that are flushed from cover by the foraging ants. This behaviour was once considered mutualistic, but later studies show that it is instead kleptoparasitic, with the birds stealing prey. Birds indulge in a peculiar behaviour called anting that is as yet not fully understood. Here birds rest on ant nests, or pick and drop ants onto their wings and feathers; this may remove ectoparasites. Anteaters, pangolins and several marsupial species in Australia have special adaptations for living on a diet of ants. These adaptations include long, sticky tongues to capture ants and strong claws to break into ant nests. Brown bears (Ursus arctos) have been found to feed on ants, and about 12%, 16%, and 4% of their faecal volume in spring, summer, and autumn, respectively, is composed of ants.

Ants perform many ecological roles that are beneficial to humans, including the suppression of pest populations and aeration of the soil. The use of weaver ants in citrus cultivation in southern China is considered one of the oldest known applications of biological control. On the other hand, ants can become nuisances when they invade buildings, or cause economic losses.

In some parts of the world (mainly Africa and South America), large ants, especially army ants, are used as surgical sutures. The wound is pressed together and ants are applied along it. The ant seizes the edges of the wound in its mandibles and locks in place. The body is then cut off and the head and mandibles remain in place to close the wound.

Some ants of the family Ponerinae have toxic venom and are of medical importance. The species include Paraponera clavata (Tocandira) and Dinoponera spp. (false Tocandiras) of South America and the Myrmecia ants of Australia.

In South Africa, ants are used to help harvest rooibos (Aspalathus linearis), which are small seeds used to make a herbal tea. The plant disperses its seeds widely, making manual collection difficult. Black ants collect and store these and other seeds in their nest, where humans can gather them en masse. Up to half a pound (200 g) of seeds can be collected from one ant-heap.

Although most ants survive attempts by humans to eradicate them, a few are highly endangered. These are mainly island species that have evolved specialized traits and include the critically endangered Sri Lankan relict ant (Aneuretus simoni) and Adetomyrma venatrix of Madagascar.

It has been estimated by E.O. Wilson that the total number of individual ants alive in the world at any one time is between one and ten quadrillion (short scale). According to this estimate, the total biomass of all the ants in the world is approximately equal to the total biomass of the entire human race.

Ants and their larvae are eaten in different parts of the world. The eggs of two species of ants are the basis for the dish in Mexico known as escamoles. They are considered a form of insect caviar and can sell for as much as USD 40 per pound (USD 90/kg) because they are seasonal and hard to find. In the Colombian department of Santander, hormigas culonas (roughly interpreted as "large-bottomed ants") Atta laevigata are toasted alive and eaten.

In areas of India, and throughout Burma and Thailand, a paste of the green weaver ant (Oecophylla smaragdina) is served as a condiment with curry. Weaver ant eggs and larvae as well as the ants themselves may be used in a Thai salad, yum (ยำ), in a dish called yum khai mod daeng (ยำไข่มดแดง) or red ant egg salad, a dish that comes from the Issan or north-eastern region of Thailand. Saville-Kent, in the Naturalist in Australia wrote "Beauty, in the case of the green ant, is more than skin-deep. Their attractive, almost sweetmeat-like translucency possibly invited the first essays at their consumption by the human species". Mashed up in water, after the manner of lemon squash, "these ants form a pleasant acid drink which is held in high favor by the natives of North Queensland, and is even appreciated by many European palates".

In his First Summer in the Sierra, John Muir notes that the Digger Indians of California ate the tickly acid gasters of the large jet-black carpenter ants. The Mexican Indians eat the replete workers, or living honey-pots, of the honey ant (Myrmecocystus).

Some ant species are considered pests, and because of the adaptive nature of ant colonies, eliminating the entire colony is nearly impossible. Pest management is therefore a matter of controlling local populations, instead of eliminating an entire colony, and most attempts at control are temporary solutions.

Ants classified as pests include the pavement ant, yellow crazy ant, sugar ants, the Pharaoh ant, carpenter ants, Argentine ant, odorous house ants, red imported fire ant and European fire ant. Populations are controlled using insecticide baits, either in granule or liquid formulations. Bait is gathered by the ants as food and brought back to the nest where the poison is inadvertently spread to other colony members through trophallaxis. Boric acid and borax are often used as insecticides that are relatively safe for humans. Bait may be broadcast over a large area to control species like the red fire ant that occupy large areas. Nests of red fire ants may be destroyed by following the ants' trails back to the nest and then pouring boiling water into it to kill the queen. This works in about 60% of the mounds and requires about 14 litres (3 imp gal; 4 US gal) per mound.